SOFIA (Reuters) - Bulgaria's defense minister said on Sunday there had been a rise in violations of its airspace by Russian military and commercial aircraft in the past month, calling the alleged breaches a "provocations toward Bulgaria and its air forces".

Nikolay Nenchev told Bulgarian television channel Nova TV that Russian military aircraft had entered what he termed "Bulgaria's area of responsibility" in NATO airspace four times in the past month.

He said the planes had turned off their transponders – communications devices that, alongside normal radars, make it easier for an airplane to be located, especially in congested air space.

"It is very worrying, so we take preventive measures," Nenchev said. Any unauthorized entry of an aircraft required the scrambling of Bulgarian fighter jets, he added.

Russian passenger planes breached the airspace six times in the past month, he said.

Bulgaria had demanded an explanation from Moscow over the violations, Nenchev said, describing the incidents as "provocations toward Bulgaria and its air forces".

The Russian Defence Ministry was not immediately able to comment when contacted by Reuters.

In January, NATO Secretary General Jens Stoltenberg urged Russia to "take all necessary measures" to ensure NATO airspace was not violated.

His comments came after Turkey shot down in November a Russian warplane flying a sortie over Syria that it said had violated its airspace, triggering a diplomatic rupture in which Russia imposed economic sanctions.

Last September, Bulgaria denied Russian aid flights bound for Syria entry to its airspace, citing it had serious doubts about the cargo onboard.

Bulgaria, a former Communist state and once staunch ally of Moscow, is almost entirely dependent on Russian energy supplies, and many Bulgarians feel a deep affinity for their giant neighbor across the Black Sea.